In an application such as a wrist telephone, where space and available power are severely limited, one is compelled to minimize the amount of circuitry used to perform the intended function. One must attempt to get as much functionality as possible from that circuitry that is included. This design philosophy was practiced in the early days of electronics--although at that time for economic reasons, where active devices, namely vacuum tubes, were at a premium. Though this philosophy is, to a large extent, incommensurate with robustness of design, it appears in this instance to be a necessary evil. In conceding, howeaer, one must be judicious in making design choices to arrive at an optimal compromise.
A microwave radio transmitter that will operate from coin or button cells will be necessarily limited to developing signal strengths in the order of only a milliwatt. Since it takes at least this much signal power to drive a microwave modulator or mixer, most local oscillators are designed to develop this much power.
For these reasons, plus to keep component count and power consumption small, and to avoid conversion loss and the need for subsequent amplification, there is strong impetus in the present application to avoid the use of such modulators. It is preferable to drive the antenna directly by the oscillator with no intervening circuitry. Even the output filter can be avoided provided the oscillator can be designed to exhibit adequate spectral purity to keep harmonics at or below an acceptable level.
To transmit at the correct frequency the oscillator, typically a voltage controlled oscillator (VCO), must be locked to a stable frequency reference. This is for removing frequency drift or uncertainty due to variations in circuit parasitics, or changes in temperature, etc. A phase locked loop (PLL), usually employed for this purpose, is essential. The PLL will also prevent the VCO frequency from de-tuning due to unavoidable changes in antenna loading that are likely to occur with a portable device.
The question arises: How do we modulate the carrier?
A. Selecting a Modulation Format
A.1 On/Off Keying: The VCO could be keyed on and off--an amplitude modulation (AM) scheme--however two problems would arise from this. First, on/off keying (OOK) leaves half of the RF (radio frequency) power in the unmodulated carrier. With such minuscule transmitted power in our application, the signal received at the base station will be extremely weak. There is great concern for the ability to detect this signal in the presence of noise therefore, we don't want to waste any. Second, each time the VCO is turned off then back on again, the PLL will need to re-acquire phase lock. This can cause the system to produce energy at undefined frequencies outside the assigned channel during re-acquisition and, depending upon how long re-acquisition takes, can severely limit the keying rate achievable.
A.2 Frequency Modulation: A VCO may easily be frequency modulated simply by applying the modulating signal to its control port. Frequency modulation (FM) and phase modulation (PM) are essentially very similar and typically yield waveforms of constant amplitude as do their digital counterparts, frequency shift keying (FSK) and phase shift keying (PSK). Unfortunately, with currently available discriminators, wideband FM results in a significantly higher symbol detection threshold than does AM--typically between 10 and 20 dB, depending upon modulation index. For a given transmitted power, this impairment translates immediately into one of reduced signaling range.
A.3 Phase Shift Keying: The most efficient modulation schemes, in terms of symbol detection threshold and bandwidth utilization are the phase shift keying (PSK) schemes. These include binary phase shift keying (BPSK), quaternary phase shift keying (QPSK), staggered or offset quaternary phase shift keying (OQPSK), minimum shift keying (MSK), and sinusoidal frequency shift keying (SFSK)--the latter four having at least a 2:1 signaling-rate-to-bandwidth advantage over BPSK. Intrinsically, they are all constant envelope modulations.
When BPSK and QPSK signals are band limited at the transmitter to reduce their excess sideband energy, their envelopes acquire amplitude variations. There is the concern that nonlinearities in the amplifiers at the receiver or limiters at repeaters can restore the constant amplitude and, along with it, the undesirable excess sideband energy that can interfere with adjacent channels. OQPSK limits inter-symbol phase transitions to .+-.90.degree., reducing the above tendency somewhat. MSK and SFSK inherently reduce the occupied spectral bandwidth by making these phase transitions continuous.
MSK and SFSK are very similar to OQPSK. They all share having their inter-symbol phase transitions limited to .+-.90.degree.. They differ from OQPSK primarily in the shaping of the applied modulation signaling bits to yield differing spectral roll-off rates. OQPSK, MSK and SFSK all exhibit the same E.sub.b /N.sub.o (energy per symbol to noise energy ratio) thresholds for a given bit error rate (BER) as BPSK and QPSK do. I shall refer to OQPSK, MSK and SFSK, as well as the remainder of the general class simply as the SQPSKs (staggered quadrature phase shift keyed).
B. Selecting a Modulation Technique
B.1 Linear Synthesis: PSK may be implemented by either linear or nonlinear modulation techniques. Most implementations that appear in the literature are synthesized as linear modulation--an AM technique. FIG. 1 indicates the construction of MSK waveforms by linear synthesis, for the bit sequence (b.sub.ev, b.sub.od) indicated and as described in the following equation: ##EQU1## where: t is time;
.omega..sub.c is the carrier frequency;
T is the input signaling bit interval;
b.sub.ev and b.sub.od are the Boolean signaling bits to be transmitted, taking on the values 0 or 1;
d.sub.1 =2b.sub.ev -1 and d.sub.Q =2b.sub.od -1, each valid for 2T seconds with the d.sub.Q transitions offset from those of d, by T seconds, are antipodal signals taking the values .+-.1; and
S.sub.am (t) is the modulated carrier.
Linear synthesized PSK as suggested by Sam(t) above would require a phase splitter to generate in-phase and quadrature (I and Q) carrier signals, followed by modulator elements (mixers), a combiner, plus an amplifier to regenerate the signal, all between the oscillator and the antenna. It is these excess elements we are seeking to avoid.
B.2 Nonlinear Synthesis: FIG. 2 indicates the construction of MSK-like waveforms by nonlinear synthesis as described in the following equation for the same data as used to generate FIG. 1. ##EQU2## where:
b.sub.1 and b.sub.2 are modulator control bits (all data is in b.sub.1 :b.sub.2 simply toggles the phase between I and Q), each valid for T seconds, derived from the signaling bit sequence listed earlier to yield the same modulated carrier phase sequence that S.sub.am (t) does in FIG. 1;
g(t) is the impulse response of the pulse shaping function--usually a lowpass filter, (note that the asterisk preceding g(t) is intended to indicate convolution); and
S.sub.pm (t) is the modulated carrier.
The waveform .PHI. (t) shown in the figure is the modulation phase resulting from the bit pair sequence {b.sub.1,b.sub.2 }, after being smoothed by the band limited response of the phase locked loop, g(t). Note the similarity of the modulated carrier waveforms S.sub.am (t) and S.sub.pm (t) in FIGS. 1 and 2 respectively.
The generation of the modulator control bits b.sub.1 and b.sub.2 is indicated in FIG. 3. For comparison, offset signaling bits d.sub.I and d.sub.Q from the previous example are shown here also.
This latter function S.sub.pm (t), implies nonlinear synthesis--an FM or PM technique--and is amenable to implementation with a direct connected VCO.